It’s an old program that converts between .deb (Debian), .rpm (RedHat), .tgz (Slackware), .slp (Stampede), .pkg (Solaris), and LSB packages.
I don’t use it much, but it can be handy in a pinch for installing software that isn’t packaged for your distribution. Just don’t use it for anything low-level or that’s already packaged natively, or you’ll break stuff.